1. Field of the Invention
The present invention relates to a drive circuit for outputting a current signal, and further relates to a display apparatus using the drive circuit.
2. Related Background Art
A display apparatus of an active matrix system using organic electroluminescent (EL) elements can light individual pixels at higher gradation in comparison with a conventional display apparatus of a simple matrix system in which light emission is controlled by performing only turning on or off operations of electrodes arranged in a lattice. Consequently, by the display apparatus adopting the active matrix system, a display having a large contrast ratio and a high response speed can be realized.
The EL display apparatus includes an image display unit arranging pixels therein, and a drive circuit for processing signal information of an image signal and the like input from the outside to transmit the processed signal information to each pixel in the image display unit. In the drive circuit, a drive control circuit to be built in the same display panel as that of the image display unit is normally configured to use a thin film transistor (TFT). Moreover, the TFT's are mainly used also as active elements for controlling light emitting states of the EL element at each pixel. However, TFT elements disperse in a large scale in their characteristics in comparison with complementary metal-oxide semiconductor (CMOS) transistors, and it is difficult to ensure the correlativity of the dispersion even in adjacent areas. Consequently, when circuits are not designed so as to control drive states surely, nonuniformity in luminance is generated even if it is tried that all of the pixels emit light uniformly.
Japanese Patent Application Laid-Open No. 2003-66865 discloses a configuration of a pixel circuit for reducing the variations of current values stored in the pixel circuit by configuring the pixel circuit using four TFT's to be controlled with a plurality of gate lines and a source line for suppressing the influences of kink currents of the transistors without adopting the source follower configuration of the transistors for controlling the currents flowing through an EL element.
A circuit disclosed in Japanese Patent Application Laid-Open No. 2002-91377, as shown in FIG. 13, includes a current detection circuit 105 for detecting a current flowing through an organic EL element 103, and an error amplification circuit 102 for amplifying a difference between an output voltage of the current detection circuit 105 and an output voltage of a sample hold circuit 101 to input the amplified difference into a current control circuit 104 in a pixel circuit. The circuit is configured to make the output voltage of the current detection circuit 105 and the output voltage of the sample hold circuit 101 equal by a negative feedback operation. Thereby, the circuit controls luminance to be uniform.
Japanese Patent Application Laid-Open No. 2002-278513 discloses a configuration shown in FIG. 14. In the configuration, current detection circuits are not provided in every pixel, but a current measurement element 110 is provided to each supply line of an power source 108. The current measurement elements 110 measure the currents of a certain row according to a control state of a scanning driver 111, and after that the measured currents are stored in storage means 109. Then, the stored currents are calculated by an arithmetic element 107 and an external data driver 106, and after that the calculated currents are fed back to image data.
As display elements, various elements are known in addition to the EL element. U.S. Pat. No. 6,195,076 discloses a configuration for driving electron emission elements by a current signal.